The present invention relates to a semiconductor manufacturing method and more particularly to a semiconductor manufacturing method for crystallinically growing a group III-V compound semiconductor using Mg as a dopant.
The present invention also relates to a semiconductor laser device manufacturing method for manufacturing an AlGaInP based semiconductor laser device by using the semiconductor manufacturing method.
In recent years, semiconductor laser devices of a group III-V compound semiconductor (e.g., AlGaInP) system as those used for DVD (Digital Versatile Disc) drives are required to have severe specifications of the capabilities of high efficiency, low operating voltage, good temperature characteristic, high power operation and so on. In order to achieve the specifications, it is required to accurately control the doping profile of the dopants in the crystal growth stage.
Although it has been general to use Zn as a p-type dopant for the group III-V compound semiconductors, it is desired to select a dopant that is hard to diffuse in order to obtain a steep doping profile. Therefore, as described in, for example, JP H06-45708 A and JP 2001-24278 A, Mg has come to be used as a p-type dopant in place of Zn.
The problems that the present invention intends to solve are described in detail.
i) Necessity of Power Increase
In order to achieve a high power with a semiconductor laser device, it is important to emit more light with a small amount of carriers injected and to reduce the absorption of oscillation light. That is, a low threshold current and high efficiency are demanded. This is because, if the efficiency is poor, a large amount of current is required for outputting same optical output, and the chip temperature of the semiconductor laser device rises, resulting in causing deterioration and difficulties in further increasing the power.
As a reason for the reduction in the efficiency, an increase in the absorption of oscillating light can be considered. That is, when the optical output is increased, part of the output is absorbed and transformed into heat, and this also raises the chip temperature of the semiconductor laser device and causes the deterioration of the semiconductor laser device.
Therefore, it is demanded that the absorption of oscillating light is a little, the efficiency is high, and crystallinity is good in the crystal growth stage. From this point of view, Mg can be enumerated as a prospect for the p-type dopant for the group III-V compound semiconductors.
ii) Dopant and Doping Profile
When dopants are diffused into the active layer, the crystalline orientation of the active layer is disordered, not only the efficiency is reduced by the degraded crystallinity of the active layer itself, but also the dopants themselves disadvantageously become absorbers. Therefore, the efficiency is further reduced, and the reliability is also lowered.
Moreover, the distribution of light expands in the neighborhood of the active layer. Therefore, when the dopants especially in nonactivated state exist more than necessary in a location close to the active layer even if the dopants are not diffused up to the active layer, the dopants become optical absorbers and cause reduced efficiency and lowered reliability.
On the other hand, when the undoped region is increased in the neighborhood of the active layer in order to prevent the dopants from diffusing into the active layer, the crystallinity of the active layer is maintained, and the absorption by the dopants is also reduced. However, it becomes difficult for the carriers to be injected into the active layer, and the efficiency is disadvantageously reduced. Moreover, the carriers easily overflow particularly in operation at an elevated temperature in this case, and the temperature characteristic deteriorates.
Therefore, it is desired that the dopants exist sufficiently and not excessively in amount to a location immediately close to the active layer and no dopant exists in the active layer. That is, it is desired that a doping profile of a steep change (almost in steps) in dopant concentration in the crystal growth direction (depthwise direction) in the neighborhood of the active layer of a semiconductor laser device (the profile referred to as a “steep doping profile”).
iii) Mg Doping Delay
If Mg is used as a dopant when crystal growth is carried out by, for example, an MOCVD (metal-organic chemical vapor deposition) method, a phenomenon that Mg is adsorbed and accumulated to a dopant supply piping (precisely a portion downstream from a valve for changeover between supply and nonsupply) between a dopant material container and a reactor (furnace for reaction) and so on (so-called the “memory effect”) occurs. Therefore, a time delay (so-called the “doping delay”) from when Mg starts being flowed from the dopant material container until when the doping is actually started in the crystals occurs. Moreover, even after the supply of Mg from the dopant material container is stopped, the Mg accumulated in the dopant supply piping flows to the reactor, disadvantageously doping the crystals.
As described above, there is a problem that it is difficult to accurately control the Mg doping profile due to the memory effect.
iv) Mg Diffusion Front
Atoms in the crystals tend to diffuse from the high concentration side to the low concentration side when there is a concentration difference. In the semiconductor laser device, it is required that the n-type and the p-type are selectively fabricated with interposition of the active layer, and it is required that the donor and the acceptor for forming the n-type and the p-type do not exist in the active layer but sufficiently exist in the n-type layer and the p-type layer. That is, a great concentration difference occurs between the active layer and the n-type layer and the p-type layer that interpose it therebetween. Diffusibility also differs depending on the type of the atoms, and diffusion is caused by the concentration difference even when Mg is used as the p-type dopant.
Therefore, it is required to control the extent of diffusion of Mg from the Mg-doped layer to the nondoped layer (this is referred to as a “Mg diffusion front”).